Guglielmi et al., J. Appl. Electrochem., 19, 167 (1989) disclose perfluorinated lithium ion exchange polymer electrolyte gels. Gels were formed by dissolving Nafion.RTM. 1100 perfluorinated ionomer resin in n-methyl formamide (NMF), propylene carbonate (PC), and triethylphosphate (TEP) are disclosed. Conductivities in excess of 10.sup.-4 S/cm were obtained. Heating of the solutions to 80.degree. C. is also disclosed, along with a concomitant increase in specific conductance to ca. 7.5.times.10.sup.-3 S/cm measured at the elevated temperature. The solvent to polymer ratio in the gels was in excess of 3:1 by weight. The gels disclosed therein are not capable of being formed into free-standing membranes.
Aldbert et al., Polymer Journal, 23, 399 (1991) disclose perfluorinated sulfonate ionomers wherein the cations are hydrogen, lithium, sodium, and rubidium, and wherein the ionomer is in the form of a solution, or a solvent-swollen membrane. A 50/50 mixture of PC and dimethoxyethane is shown to be particularly effective at increasing the conductivity of a gel solution formed therefrom with lithium ionomer form of Nafion.RTM. 1100. Membranes formed of the lithium ionomer form of Nafion.RTM. 1100 are swollen with PC, NMF, and ethanol. It is disclosed that after ca. 10 minutes of solvent soaking at room temperature, the specific conductivity of the membrane exhibits no further increase. Specific conductivity as high as 1.8.times.10.sup.-3 S/cm is found for a membrane soaked in NMF for less than 10 minutes. Membranes swollen in PC provide a specific conductivity of 6.times.10.sup.-5 S/cm. The former solvent cannot be used in lithium batteries due to its protic nature. Propylene carbonate is known in the art to be a good lithium battery solvent, but the conductivity disclosed by Aldbert et. al. in Nafion.RTM. 1100 membranes is less than 10.sup.-4 S/cm, too poor to be of practical utility. Solvent to polymer ratio in the solvent swollen membranes was less than 1:1.
In Zawodzinski et al., J. Electrochem. Soc., 140, 1041 (1993) a Nafion.RTM. 1100 membrane is heated to 105.degree. C. prior to exposure to water at room temperature. The effect of the heating is to prevent the achievement of maximum water absorption, thus decreasing the conductivity of the water-swollen membrane.
M. B. Armand, "Ionically Conductive Polymers", Solid State Batteries, Sequeira and Hopper, eds. (1985), discloses the lithium ionomer form of Nafion.RTM. 1100 soaked in PC for 48 hours, developing a specific conductivity of 2.1.times.10.sup.-5 S/cm. Armand also discloses that the conductivity of the solvent soaked membrane increases with temperature according to a free volume model. As with Aldbert et al., the conductivity disclosed here is too poor to be of practical utility in commercial lithium batteries.
Miura et al, Memoirs of Faculty of Tech. Tokyo Metropolitan Univ., No. 40, pp 4349ff, 1990, describe metastable changes in the ion cluster morphology of perfluorinated ionomers upon swelling in ethanol followed by drying.
Gebel et al, Polymer, v. 34, pp 333ff, 1993, disclose perfluoro lithium ionomer membranes swollen in numerous solvents including propylene carbonate and phosphates. Phosphates are disclosed to provide particularly large amounts of swelling. The necessity of soaking in some solvents, such as propylene carbonate, for a period of weeks is disclosed. There is no discussion of ionic conductivity.
Perron et al., Specific Conductivity of Lithium bis(trifloromethane-sulfonyl)imide (LiTFSI) in Mixtures of Aprotic Solvents, Abstract 64 of Battery/Energy Technology Joint General Session of Meeting Abstracts, Electrochemical Society May 4-9, 1997 meeting, Montreal, Quebec, Canada, disclose dimethoxyethane (DME) as a good solvent for lithium salts providing high specific conductivity to solutions thereof. Solutions of non-polymer lithium salts in certain combinations of aprotic solvents are disclosed to provide particularly high specific conductivity in cases of limited solubility of the electrolyte membrane in the solvent which affords higher conductivity. Mixtures of DME with propylene carbonate are disclosed. Small amounts of PC do not significantly alter the conductivity of an electrolyte solution in DME.
Nonaqueous liquid electrolytes for lithium batteries are disclosed in Okuno et al, U.S. Pat. No. 5,525,443. Disclosed are lithium salt solutions formed with solvent combinations of linear and cyclic esters the cyclic ester being selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, and gamma-butyrolactone, and the linear ester being selected from the group consisting of diethyl carbonate, dimethyl carbonate, ethyl formate, methyl formate, ethyl acetate, methyl acetate, and dimethyl sulfoxide.